The second has for its pair of appendages the small pair of limbs which in all living Arachnids is either chelate or retrovert (as in spiders), and is known as the chelicerae.
Although the prae-oral pair of appendages in the higher Arachnida is usually chelate, it is not always so; in spiders it is not so; nor in many Acari.
The reduction of the organism to seven leg-bearing somites, of which the first pair, as in so many Eu-arachnida, are chelate, is a form of degeneration connected with a peculiar quasi-parasitic habit resembling that of the crustacean Laemodipoda.
The first pair of limbs is often chelate or prehensile, rarely antenniform; whilst the second, third and fourth may also be chelate, or may be simple palps or walking legs.
Appendages of 1st pair.tri-segmented, chelate; of 2nd pair chelate, with their basal segments subserving mastication; of 3rd, 4th, 5th and 6th pairs similar in form and function, except that in recent and Carboniferous forms the basal segments of the 3rd and 4th are provided with sterno-coxal (maxillary) lobes, those of the 4th pair meeting in the middle line and underlying the mouth.
Appendages of 2nd pair folding in a horizontal plane, completely chelate, the claw immovably united to the sixth segment.
Appendages of 2nd pair folding in a vertical plane, not chelate, the claw long and movable.
Appendages of 1st pair consisting of three segments, completely chelate, without poison gland; of 2nd pair slender, leg-like, tipped with three claws, the basal segment without sterno-coxal process taking no share in mastication, and widely separated from its fellow of the opposite side; 3rd, 4th, 5th and 6th appendages similar in form to the 2nd and to each other.
Appendages of 1st pair large, chelate, bisegmented, articulated to the sides of the head-shield; appendages of 2nd pair simple, pediform, with protrusible (?
Pair bisegmented completely chelate, furnished with peculiar organs, the serrula and the lamina.
Appendages of 2nd pair very large and completely chelate, their basal segments meeting in the middle line, as in the Uropygi, and provided in front with membranous lip-like processes underlying the proboscis.
Appendages of 2nd pair, with their basal segments uniting in the middle line below the mouth, weakly chelate at apex.
The first tribe, called Chelifera, from the usually chelate or claw-bearing first limbs, may be regarded as Isopoda anomala, of whieh some authors would form a separate order, Tanaidea.
This chelate condition may be assumed by almost any of the appendages, and sometimes it appears in different appendages in closely related forms, so that no very great phylogenetic importC ance can in most cases be attached to it.
Such chelate rami or limbbranchesare independently developed in Crustacea and inArachnida, and are carried by somites of the body which do not correspond in position in the two groups.
The range of modification of which the rami or limb-branches of the limbs of Arthropoda are capable is very large, and in allied orders or even families or genera we often find d z what is certainly the palp of the same appendage (as determined by numerical position of the segments) - in one case antenniform, in another chelate, in another pediform, and in another reduced to a mere stump or absent altogether.
Head tritognathous and diprosthomerous - that is to say, with two prosthomeres, the first bearing typical eyes, the second a pair of appendages reduced to a single ramus, which is in more primitive forms antenniform, in higher forms chelate or retrovert.